That's because young dinosaurs didn't look like Mini-Me versions of their parents, according to new analyses by paleontologists Mark Goodwin, University of California, Berkeley, and Jack Horner, of Montana State University.

Instead, like birds and some other living animals, the juveniles went through dramatic physical changes during adulthood.

This means many fossils of young dinosaurs, including T. rex relatives, have been misidentified as unique species, the researchers argue.

How T. Rex Became a Terror

The lean and graceful Nanotyrannus is one strong example. Thought to be a smaller relative of T. rex, the supposed species is now considered by many experts to be based on a misidentified fossil of a juvenile T. rex.

The purported Nanotyrannus fossils have the look of a teenage T. rex, Horner said in the new documentary. That's because T. rex's skull changed dramatically as it grew, he said.

The skull morphed from an elongated shape to the more familiar, short snout and jaw, which could take in large quantities of food.

But the smoking gun, Horner said, was the discovery of a dinosaur between the size of an adult T. rex and Nanotyrannus.

Nanotyrannus—actually a young T. rex in Horner's view—had 17 lower-jaw teeth, and an adult T. rex had 12.

The midsize dinosaur had 14 lower-jaw teeth—suggesting that it was also a young T. rex, and that tyrannosaurs gradually traded their smaller, blade-like teeth for fewer bone-crushing grinders in adulthood.